An articulated road vehicle comprises a tractor unit to which can be coupled a trailer. The tractor unit is generally fitted with a rear view mirror on each side of the driver's cab. When driving in a straight path, these mirrors provide adequate visibility to the rear corners of the trailer. However, when the tractor unit is rotated relative to the trailer, either when reversing or whilst negotiating a tight turn in the forward direction, the rear corners of the trailer are outside the field of view of the mirrors. In order to combat this problem, an angle sensing apparatus may be arranged on the tractor to vary the position of the rear view mirrors according to the angle between the tractor and the trailer so that the driver can always see, using the mirror, along the length of the trailer. Such apparatus is disclosed in EP-A-094817, U.S. Pat. No. 3,950,080, U.S. Pat. No. 4,609,265, U.S. Pat. No. 4,679,158; WO88/04619.
One type of angle sensing device is described and claimed in our GB patent 2259064B and includes a lever connected to a potentiometer mounted on the tractor, the potentiometer acting as a sensor, adjacent to the coupling of the tractor and trailer, and an elastic member extending from the lever which can be detachably connected to a mounting point on the trailer. Information about the angle of rotation of the trailer relative to the tractor is provided to a control unit in the cab of the tractor unit which in turn actuates drive means to rotate the mirror accordingly about a generally vertical axis. In many cases it is preferable to mount the sensor to the rear of the coupling point and detachably mount the elastic member on a convenient point under the trailer. It has been discovered that in different sensor mounting positions there is a risk of damage to the sensor as the trailer is coupled or decoupled. It is accordingly one object of the invention to provide means for mounting an angle sensor on the tractor unit of an articulated road vehicle which reduces the risk of damage to the sensor at least during the coupling and decoupling of the trailer unit.